The Sr/Ca ratios in calcium carbonate fossils are used by the paleooceanographic community to infer past environmental conditions, such as sea surface temperature and ocean chemistry. The processes of biogenic calcification that produce these chemical signatures are complex and not fully understood, however, and vital effects are known to affect the trace element composition of the CaCO₃ biomineral products. The recent discovery that calcifying organisms produce amorphous calcium carbonate (ACC) as an intermediate phase during the crystallization process calls into question whether this pathway to mineral formation affects trace element distributions in the final product. This non-classical mineralization process raises the question of whether the Sr/Ca ratios of the final products are dependent upon temperature. That is, what is the temperature dependence of Sr/Ca ratios in calcite produced via ACC compared to the measurements obtained from calcite grown by the classical process in laboratory experiments and from biogenic settings. The goal of this study is to determine the effects of solution chemistry and temperature on the Sr composition of ACC and resultant crystalline CaCO₃. Two types of experiments were designed: First, experiments were conducted to synthesize inorganic ACC in a batch reactor for a suite of selected chemical compositions and allowing this intermediate phase to transform into calcite in the reactant solution. In a second series of experiments, ACC was precipitated by a flow-through method to compare results to the batch reactor experiments. The experimental design focused on determining the Sr/Ca ratio and Sr distribution coefficients (KD, Sr) of the amorphous and final crystalline products. Mg/Ca ratios of 5/1 were found to suppress Sr uptake into ACC by a factor of 25% when the initial Sr solution had concentration of one millimolar. ICP-AES data collected across the 18° to 30°C range showed that the Sr/Ca ratio in both ACC and the resultant calcite was independent of temperature. Upon transformation, the Sr/Ca ratios of both the ACC and calcite product were found to be similar, showing that Sr/Ca ratios were independent of the transformation process. Analysis of the data determined KD, Sr values of 0.564(±0.006) for ACC and 0.466(±0.009) for the resultant calcite in the 18-30°C temperature range. The findings show that the Sr/Ca ratios of ACC and the transformed calcite are independent of temperature. However, the corresponding KD, Sr values exceed those reported for calcite grown by classical processes by an order of magnitude. The findings for the inorganic calcite yield KD, Sr values up to four times higher than those found in biogenic calcites. Because the findings of this study show that Sr/Ca is independent of temperature, this study calls into question whether previously reported Sr/Ca measurements in biogenic calcites should be revisited. It is plausible that biological factors have a significant influence on trace element incorporation into biogenic calcite. Vital effects, such as the influence of macromolecules during the ion uptake process, may regulate the apparent Sr/Ca versus temperature trends observed in marine paleontology. Higher KD, Sr values in marine calcifiers may indicate that organisms use the non-classical mineralization pathway in whole or in part. Future studies of trace element incorporation in calcifying species should consider the pathway to mineralization in tandem with interpretations of environmental controls on distribution coefficients. / Master of Science

Carbonate geochemistry plays an important role in understanding environmental conditions during the time of precipitation. The studies for this dissertation research were focused on carbonate precipitation and crystallization in different chemical and physical environments. The first project aimed to precipitate aragonite at low oxygen levels to identify a correlation between partitioning of trace elements and anoxic and suboxic conditions. The second study focused on the precipitation of amorphous calcium carbonate in varying magnesium concentrations to determine the identity of crystalline material after transformation of ACC. Lastly, the third project was developed to understand transformation of CaCO3 polymorphs. Specifically, whether or not geochemistry is retained from one polymorph to the next. All projects could aid in development of paleoproxies to be used for determining past environmental and climatic conditions in the past.

La précipitation et la dissolution du carbonate de calcium (CaCO3) sont des processus clés dans les systèmes naturels en raison de leur association intime avec le cycle du carbone terrestre. La précipitation se produit généralement sur des substrats étrangers en abaissant les barrières énergétiques qui contrôlent la nucléation. Ce processus appelé nucléation hétérogène résulte d'une interaction entre la sursaturation du fluide et les différentes énergies d’interface entre substrat-noyau-fluide. Malgré l’importance des énergies d’interface sur le devenir de la nucléation hétérogène, la littérature actuelle reste rare dans leurs valeurs absolues, limitant la précision de la modélisation du transport réactif. La formation des biominéraux constitue un réservoir majeur des carbonates dans la lithosphère. Des études récentes ont révélé des nucléations par multi-étapes impliquant la formation du carbonate de calcium amorphe (ACC), un intermédiaire métastable durant les premiers stades de la formation des biominéraux. De tels précurseurs amorphes permettent de réaliser les formes complexes des biominéraux, tandis que leur stabilité et leur cinétique de cristallisation sont contrôlées par de multiples facteurs. L'élucidation des mécanismes sous-jacents est bénéfique pour le développement de matériaux biomimétiques.Le premier objectif est de développer une compréhension prédictive des valeurs d'énergie d’interface régissant la nucléation hétérogène du CaCO3 en fonction des propriétés physico-chimiques spécifiques des substrats, comme l'hydrophobicité. Cette dernière est étudiée en utilisant de la phlogopite avec et sans substitution par le fluor produisant des substrats hydrophobes et hydrophiles. La technique de diffusion des rayons X aux petits angles en incidence rasante a été employée in situ pour obtenir des valeurs d’énergie effective d’interface. Il est intéressant de noter que les valeurs extraites pour les deux substrats sont similaires, et thermodynamiquement les deux fournissent un bon modèle pour la nucléation, alors que leurs mécanismes sont différents. La caractérisation ex situ par microscopie à force atomique a montré que le substrat hydrophile favorise la formation et la stabilisation d’ACC, tandis que le substrat hydrophobe favorise la formation de calcite. Ces résultats soulignent la flexibilité structurelle intrinsèque du CaCO3 et son avantage dans les processus de nucléation hétérogènes.Le deuxième objectif est de fournir une description atomistique de l'hydrophobicité du substrat. L'adsorption d'eau sur la phlogopite a été réalisée in situ par spectroscopie de photoélectrons à pression ambiante pour étudier l'effet de la substitution par le fluor et de différents types de contre-ions (K+, Na+ vs. Cs+). Ces résultats ont été interprétés par des simulations de dynamique moléculaire et la théorie de bond-valence. La combinaison de ces techniques montre que l'hydrophobicité du substrat provient d'une compétition entre deux facteurs: l'hydratation des contre-ions par rapport à celle du substrat.Le but final est d'étudier les mécanismes moléculaires par lesquels Mg2+, une impureté chez les précurseurs amorphes biogéniques, augmente la persistance cinétique d’ACC. La technique de diffusion inélastique incohérente des neutrons a été combinée avec la spectroscopie de corrélation de photons X pour élucider la dynamique à l'échelle nanométrique de l'eau et des ions dans les ACC. Les résultats montrent que la présence de Mg2+ augmente la diffusion atomique dans le solide tout en amplifiant la rigidité du réseau des liaisons hydrogène. Ces résultats contre-intuitifs sont abordés en considérant différents facteurs cinétiques inclus dans l’équation décrivant le taux de nucléation au sein de la théorie classique de la nucléation. Dans l'ensemble, ces résultats indiquent l'importance de l'eau comme stabilisant cinétique de la structure amorphe et de l'existence de barrières stériques qui abaissent le taux de cristallisation. / Precipitation and dissolution of calcium carbonate (CaCO3) are key processes in both natural and engineered systems due to their intimate association with the Earth’s carbon cycle. Precipitation usually occurs on foreign substrates since they lower the energetic barriers controlling nucleation events. This so-called heterogeneous nucleation results from the interplay between the fluid supersaturation and the interfacial free energies present at the substrate-nucleus-fluid interfaces. Despite the relevance of interfacial energies for the fate of heterogeneous nucleation, the current literature remains scarce in their absolute values, which limits the accuracy of reactive transport modelling. Of particular relevance to the carbon cycle, the formation of biominerals accounts for a major reservoir of the carbonate minerals in the lithosphere. Recent studies have revealed the existence of multistep nucleation pathways that involve formation of amorphous calcium carbonate (ACC), a metastable intermediate during the early stages of biomineral formation. Such amorphous precursors allow molding of the intricate shapes of biominerals, while their stability and crystallization kinetics are effectively controlled by multiple factors. Elucidating the underlying mechanisms is beneficial for the development of biomimetic materials.The first goal of this dissertation is to develop a predictive understanding of interfacial energy values governing CaCO3 heterogeneous nucleation as a function of specific physico-chemical properties of the substrates, such as hydrophobicity. This last was investigated using phlogopite, a common mica, with and without fluorine substitution yielding hydrophobic and hydrophilic substrates. In situ time-resolved Grazing-Incidence Small Angle X-ray Scattering experiments were performed to obtain effective interfacial energy values. Interestingly, the extracted values for both substrates were similar, and thermodynamically these substrates provide a good template for nucleation, but the pathways differ. By ex situ Atomic Force Microscopy characterization, the hydrophilic substrate was shown to promote the formation and stabilization of ACC, whereas the hydrophobic one favored the formation of calcite. These results point to the intrinsic structural flexibility of CaCO3 and its advantage in heterogeneous nucleation processes.The second goal is to provide an atomistic description of the substrate hydrophobicity/hydrophilicity. Water adsorption on phlogopite was studied in situ using Near-Ambient Pressure X-ray Photoelectron Spectroscopy to investigate the effect of fluorine substitution and the influence of different types of counterions (K+, Na+ vs. Cs+). The results of the spectroscopy experiments were further interpreted using molecular dynamics simulations and bond-valence theory. The combination of these techniques shows that the substrate hydrophobicity stems from a competition between two factors: hydration of counterions vs. that of substrate.The final goal is to study the molecular mechanisms by which Mg2+, a common impurity in biogenic amorphous precursors, increases the kinetic persistence of ACC. Inelastic Incoherent Neutron Scattering and X-ray Photon Correlation Spectroscopy were combined to elucidate the nanoscale dynamics of water and ions within ACC. The presence of Mg2+ was shown to enhance the atomic diffusion within the solid while simultaneously increasing the stiffness of the hydrogen bond network. These counter-intuitive results are addressed by considering the different factors included in the pre-exponential term of the nucleation rate equation within the framework of the classical nucleation theory. Overall, the results point to the importance of water as a kinetic stabilizer, and to the existence of steric barriers that lower the crystallization rate.

During the past decades, emission of greenhouse gases has accelerated to unsustainable levels. This is a serious issue that can have a devastating impact on everything from global economy to the terrestrial or marine ecosystem. A method for reducing the emission is named carbon capture and storage, which this project is based on. In this study, different concentrations of citric acid (CA) is used (as an additive) for the enhancement and optimization of carbon dioxid sorption properties of amorphous calcium carbonate (ACC), mesoporous magnesium carbonate (MMC) and calcium magnesium carbonate composite (CMC). These materials were heat treated in a calcination and an alternating carbonation process in order to study the carbon dioxid sorption performance. During the calcination process, CA undergoes a pyrolysis reaction in order to increase the specific surface area of the individual nanoparticles, which is an important factor for the sorption capacity. In the case of CMC, different molar ratios of magnesium oxide and calcium oxide were used in order to alter the concentration of the resulting magnesium oxide prior to heating. All three materials consisted of aggregations of nanometer-sized particles. Thermogravimetric analysis, scanning electron microscopy, surface area and porosimetry and infrared spectroscopy analysis suggest that the carbon dioxid sorption properties and the sintering stability of ACC and MMC do not improve since CA evaporates due to pyrolysis. Sintering was a greater problem for the evaluated CA treated ACC sample. However, in the case of CMC, the sorption and sintering properties were enhanced due to the higher Tamman-temperature of magnesium oxide, specifically for the lower concentration of magnesium oxide. After 19 carbonation cycles, CMC-1:1-25% CA showed signs of improved sintering stability and sorption capacity, compared to ACC-75% CA. / <p>Presentationen genomfördes på distans.</p>